Forces from history
Many Military Forces have come and gone during the last 4000 years. Many armies and units have fought for a period of history and achieved recognition that has lasted for many centuries, well beyond the death of their last member, through to modern times. The last Roman Legions disappeared nearly 1600 years ago and we only have historical records and archeology to understand how the lived, fought and died.
Other forces have evolved naturally into modern units. The Royal Flying Corps evolved into the Royal Air Force while the Union Army of the American Civil War evolved into the modern U.S. Army.
Some forces were created by leaders to fight their wars, and did not last beyond the demise of their creator. The Waffen SS Divisions of Hitlers Germany were destroyed at the end of World War 2 but some members still survive.
Other forces have disappeared at the end of a campaign. The Afrika Corps was effectively destoryed by the Allies in Tunisea in May 1943. The British 'Desert Rats' were also seen as veterans of World War 2, but they reappeared with new men, new equipment and new ways of fighting during the Gulf War of 1990 - 1991.
Other formations have only existed briefly to use a certain weapon type. The British Glider Pilot Regiment was phased out as Helicopters became more effective. The Parachute Regimant has remained as an elite British force, but they are unlikely to deploy en masse by parachute in the future. At the other extreme, cavalry forces have existed for thousands of years, and have nearly been phased out by the armies of the world, but not quite.